Thermionic valve circuit



Sept. 12, 1939. N. H. CLOUGH THERIIONIC VALVE CIRCUIT ARRANGEMENT Fij9v 9, 1936 Lg Y 2 II +1 mu m 31 C'arrir Input Television 5/ 9nal Inpui OSCILLATOR INVENTOR NEWSQME H NRY CLOUGH BY 1 Z Bela Asian 549/701 [up uJ ATTORN EY Patented Sept. 12, 1939 UNITED STATES I PATENT OFFICE THERMIONIC VALVE CIRCUIT ARRANGEMENT Newsome Henry Clough, Brentwood, England, as-

signor to Radio Corporation of America, a corporation of Delaware 7 Claims. (Cl. 179 -1715) This invention relates to thermionic valve circuit arrangements and more particularly to thermionic valve circuit arrangements of the kind required to handle relatively high signal frequencies and wherein the circuit is such that the cathode or cathodes of a valve or valves therein is or are necessarily at relatively high frequency potential difference with respect to earth.

It is the aim and object of this invention to provide method and means for improved operation of modulating systems. 1

In the drawing, Fig. 1 shows schematically a modulating circuit for facilitating the explanation of the invention, and

i Figs. 2 and 3 show schematically practical embodiments of the invention.

In order that the invention may be the better understood there will first be described with reference to the accompanying Figure 1 a well known modulation circuit arrangement suitable for use for television transmission and to which the present invention may be applied with considerable advantage.

In the well known television modulation arrangement just referred to, and shown in Figure 1, television signals are applied, e. g., via a coil l between grid.2 and cathode 3 of a modulating valve 4 whose filament 3 is connected to earth (or to some point at steady potential with respect to earth) through a resistance 5 (which may be termed the modulating resistance), the voltage across which is applied to efiect modulation at the grids 6, l of a succeeding carrier frequency amplifier. In the arrangement illustrated the carrier frequency amplifier consists of two valves 8, 9, in push pull and having a common cathode point ID, the two grids 5, 1, being connected togather through a carrier frequency tuned circuit l I, I2, shunted by a resistance l3. For television transmission a high carrier frequency is generally requ red and the tuned circuit H, I2, is tuned to this high frequency-for example a frequency of the order of 50 megacycles-the purpose of the shunt resistance l3 beng to provide a ballast load to prevent the tuned circuit voltage from varying appreciably under varying conditions of grid current loading due to modulation. Carrier frequency from a suitable "drive source (not shown) is appl ed to the tuned circuit, e. g., by a coil (not shown) coupled to coil II and the midpoint I4 of the coil II (or of the resistance i3) is connected to the live end of the modulating resistance 5 either directly or through some form of coupling arrangement capable of passing the modulating frequencies whilst presenting a low capacity to earth, e. g., as shown a copper oxide or similar rectifier arrangement I 5 or a float- 'ing source of steady potential or a resistance network. As will be. seen the modulating signal potentials set up across the modulating resistance 5 are applied to the grids 6, I, of the push-pull connected valves 8, 9, so as to effect modulation. With a known circuit arrangement as shown in Figure 1 difficulties are often experienced in connection with the energizing of the filament of the modulating valve 4 since the capacity to earth of the filament energizing means interferes with proper modulation and in particular tends to shunt away the higher modulating frequencies.

In accordance with this invention as applied to the above described known arrangement the above mentioned defect is substantially over- -come by energizing the filament of the modulating valve by means of high frequency energy, the frequency of which is greater than that of the highest modulating frequency employed in order not to cause interference (e. g., by "magnetron effect) with the normal operation of the modulator. Preferably the heating frequency is greater than twice the highest modulating frequency so that any beats set up between these two fre quencies will lie outside the frequency spectrum to be passed by the modulator.

The cathode heating or filament energizing high frequency current may be obtained in any convenient way. For example in the embodiment of the invention shown in the accompanying Figure 2 it is obtained from a thermionic valve oscillation generator represented by the rectangle l6 having-a tuned output circuit I! to which is coupled a second tuned circuit l8 of which the filament 3 forms part. With this arrangement and by means of the tuned circuits ll, [8, it is possible to obtain sufiicient energy to heat the filament 3 while still having only a loose coupling between the said tuned circuit I1, l8, so that there need only be quite small capacity coup-ling between them. In this way the shunting to earth of the higher frequency modulating potentials is greatly reduced although one point in the oscillator tuned circuit I! may be earthed or maintained at a steady potential with respect to earth. For example, to take a practical case of a television transmitter operating on a carrier frequency of say, megacycles, and modulated with signals lying within the frequency band extending from O to 1.5 megacycles, the modulator filament 3 may be heated by means of a current of, say, 4 megacycles frequency. For these values of frequency any beats set up between the heating current and the highest modulating frequency will be of frequency 4:1.5 megacycles, i. e., 5.5 megacycles or 2.5 megacycles, and both these beat frequencies lie outside the modulating frequency range which extends from 0 to 1.5 megacycles. The tuned circuit I8 of which the filament forms part may be such that the product of the inductance into the capacity thereof is approximately 16 10 (expressed in C. G. S. electro-magnetic units), and this permits of the use (as shown in Figure 2) of two condensers I8C (each of .0016 microfarad) in series with a total inductance (made up of the filament inductance and the inductance of a coupling coil I8L) of 2 microhenries, the two ends of the coupling coil I8L being connected to the two ends of the filament 3, each to each, through connections each of which includes one of the said two condensers IBC. Preferably an equalizing resistance I8R is shunted directly across the filament 3 in order to minimize undesired effects of alternating current heating as well known per se, the mid point of this resistance being connected to earth (or to some point of steady potential with respect to earth) through the modulating resistance 5, The coupling coil I8L is coupled to a coil IIL in the tuned output circuit of the valve oscillator I6 providing the required 4 megacycles heating current. Preferably in order to minimize earth capacity no provision is made for fine tuning in the circuit l8 but instead the two tuned circuits I'I, I8, are matched by adjusting a variable condenser I'IC forming part of the oscillator tuned circuit. Preferably also the coupling between these two tuned circuits I'I, I8, is made so that fine control of the modulator filament potential may be obtained (without introducing more earth capacity) by adjusting this coupling.

It will be appreciated that if there is unbalanced capacity from the two leads of the cathode 3 to ground, there would be an unequal fiow of current through the condensers I to ground through the distributed capacity from the cathode to the ground. However, by provision of the condensers I8C being made variable, the impedance of each condenser can be individually varied so that the total impedance of the variable condenser and the distributed capacity from the upper end of the resistanceto ground can be made to be equal to the sum of the impedances of the lower condenser I8C and the distributed capacity of the cathode 3 connected to the-lower end of the resistance I8R. When these impedances are made equal, equal current will fiow to ground, and accordingly, there will be no unbalancing of the high frequency current and as a result, there will be no spurious or cross-modulation.

'Another'way, illustrated in the accompanying Figure 3, of obtaining the high frequency energy for filament energization consists in utilizing the carrier frequency oscillations from the tuned circuit II, I2, connected between the grids of the push-pull valves at which modulation is effected. This method of obtaining the high frequency energy for filament heating is only practical so long as the carrier frequency is not so high as to cause excessive voltages to occur across the filament before the desired heating is reached, and would not normally be applicable to television transmitters operating at the high frequency already mentioned. With this arrangement the filament circuit acts as a resistive load on the tuned circuit I I, I2, and it is possible so to design the whole circuit that this resistive load replaces (either wholly or in part) the resistance I3 of Figures 1 and 2.

In other words, it is possible so to design the whole circuit that the necessary shunt resistance for the carrier frequency tuned circuit II, I2, is constituted (either wholly or in part) by resistance thrown back into the said tuned circuit by the filament circuit. The tuned circuit I 8 of which the filament forms part is only loosely coupled to the tuned circuit II, I2, so that capacity coupling between these two tuned circuits is at a minimum.

Although the invention has been specifically described in connection with its application to a known television modulation circuit arrangement, and although an important application of the invention is to television modulation circuit arrangements, the said invention is not limited to such applications, but may be applied to all cases where relatively high frequencies are to be handled by valves whose cathods are necessarily at a relatively high frequency potential difference with respect to earth, e. g., to so-called series modulation circuits, and to amplifiers of high or television frequencies whether or not they are associated with radio transmitters.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed I declare that what I claim is:

1. Modulating system comprising a source of radio frequency energy, a radio frequency amplifier, means to supply carrier wave energy from the source to said radio frequency amplifier, a thermionic amplifier having a cathode, control electrode and output electrode, a surface of fixed potential, a source of potential for energizing the thermionic amplifier, said source being connected between the output electrode and the fixed potential surface, an impedance connected between the cathode of the thermionic amplifier and the fixed potential surface, means to supply energy from the impedance to the radio frequency amplifier to control the radio frequency amplification, and input means for electrically balancing the cathode with respect to the surface of fixed potential and. for supplying radio frequency to the cathode to heat the same.

2. Modulating system comprising a source of radio frequency energy, a radio frequency amplifier, means to supply carrier wave energy from the source to said radio frequency amplifier, a thermionic amplifier having a cathode, control electrode and output electrode, a surface of fixed potential, a source of potential for energizing the thermionic amplifier, said source being connected between the output electrode and the fixed potential surface, a resistor connected between the cathode of the thermionic amplifier and the fixed potential surface, a connection from the common junction point of the resistance and the cathode to the radio frequency amplifier to control the radio frequency amplification, and input means for electrically balancing the cathode with respect to the surface of fixed potential and for supplying radio frequency to the cathode to heat the same.

3. Modulating system comprising a source of radio frequency energy, a radio frequency amplifier, means to supply carrier wave energy from the source to said radio frequency amplifier, a thermionic amplifier having a cathode, control electrode and output electrode, a surface of fixed potential, 'a source of potential for energizing the thermionic amplifier, said source being connected between the output electrode and the fixed potential surface, an impedance connected between the cathode of the thermionic amplifier and the fixed potential surface, means to supply energy from the impedance to the radio frequency amplifier to control the radio frequency amplification, a second source of radio frequency energy, input means for electrically balancing the oathode with respect to the surface of fixed potential and for supplying radio frequency to the cathode to heat the same, and means to supply energy from the second source of radio frequency to the balancing input means.

4. Modulating system comprising a source of radio frequency energy, a radio frequency amplifier, means to supply carrier wave energy from the source to said radio' frequency amplifier, a thermionic amplifier having a cathode, control electrode and output electrode, a surface of fixed potential, a source of potential for energizing the thermionic amplifier, said source being connected ,between the output electrode and the fixed potential surface, an impedance connected between the cathode of the thermionic amplifier and the fixed potential surface, means to supply energy from the impedance to the radio frequency amplifier to control the radio frequency amplification, input means for electrically balancing the cathode with respect to the surface of the fixed potential and for supplying radio frequency to the cathode to heat the same, and means to supply energy from the source of radio frequency to the balancing input means.

5. Modulating system comprising a source of radio frequency energy, a radio frequency amplifier, means to supply carrier wave energy from the source to said radio frequency amplifier, a thermionic amplifier having a cathode, a control electrode and output electrode, a surface of fixed potential, a source of potential for energizing the thermionic amplifier, said source being connected between the output electrode and the fixed potential surface, an impedance connected between the cathode of the thermionic amplifier and the fixed potential surface, means to supply energy from the impedance to the radio frequency amplifier to control the radio frequency amplification, a second source of radio frequency energy, input means for electrically balancing the oathode with respect to the surface of the fixed potential and for supplying radio frequency to the cathode to heat the same means to supply energy from the second source of radio frequency to the balancing input means, a source of, modulating potentials, and means to supply the modulatin potentials to the control electrode of the thermionic amplifier.

6. Modulating system comprising a source of radio frequency energy, a radio frequency ampli fier, means to supply carrier wave energy from the source to said radio frequency amplifier, a thermionic amplifier having a cathode, a control electrode and output electrode, a surface of fixed potential, a source of potential for energizing the thermionic amplifier, said source being connected between the output electrode and the fixed potential surface, an impedance connected between the cathode of the thermionic amplifier and the fixed potential surface, means to supply energy from the impedance to the radio frequency amplifier to control the radio frequency amplification, input means for electrically balancing the cathode with respect to the surface of the fixed potential and for supplying radio frequency to the cathode to heat the same means to supply energy from the source of radio frequency to the balancing input means, a source of modulating potentials, and means to supply the modulating potentials to the control electrode of the thermionic amplifier.

7. Modulating system comprising a source of radio frequency energy, a radio frequency amplifier, means to supply carrier wave energy from the source to said radio frequency amplifier, a thermionic amplifier having a cathode, control electrode and output electrode, a surface of fixed potential, a source of potential for energizing the thermionic amplifier, said source being connected between the output electrode and the fixed potential surface, an impedance connected between the cathode of the thermionic amplifier and the fixed potential surface, means to supply energy from the impedance to the radio frequency amplifier to control the radio frequency amplification, input means for electrically balancing the cathode with respect to the surface of the fixed potential and for supplying radio frequency to the cathode to heat the same, a source of modulating potentials, and means to supply the modulating potentials to the control electrode of the thermionic amplifier.

NEWSOME iiENRY CDOUGH. 

